With the introduction of PHP 5.3, we are providing you with the ability to upload your own user.ini files, which are used to modify PHP configuration settings on your account.
If you've used a php.ini file on a linux server, this is the same concept. You simply create a text file named user.ini in the root of your account. The file can include PHP settings such as:
upload_max_filesize = "200M"
date.timezone = "Europe/London"
These settings will be changed on your site only. While most of these settings are safe to change, the user.ini file does give you a lot of control over how PHP works for your site, so use your best judgement as to what you wish to change.
If you find that you've made a change that causes problems with your site, simply remove the line(s) causing trouble and re-upload the file. Note that there is a five minute server cache for user.ini changes, so you may not see your changes until a few minutes after uploading.